1. Field of the Invention
The present invention relates to a color cathode ray tube which is preferably used for broadcasting monitor. More particularly, it relates to a color cathode ray tube which is effective for adjusting white color balance.
2. Description of the Prior Art
In the case of adjusting white color balance of a color cathode ray tube for a video monitor used in broadcasting stations, the white color displayed on the screen of the color cathode ray tube should be precisely conformed with the predetermined reference white color. Otherwise, the images displayed on the screen impart different hue depending upon a broadcasting or a channel.
The reference white color for video monitors used in broadcasting stations are decided to be the specific white color having the predetermined color temperature. For example, in U.S.A and European countries, the reference white color is decided to a solar ray color D6500 (6550.degree. K.+7MPCD) which is near the reference C light source (color temperature of 6740.degree. K.) as the reference white color in the NTSC system or to the reference white color for the receivers (9300.degree. K.+27MPCD).
In Japan, the reference white color for video monitors is decided to D9300 (9300.degree. K.+8MPCD) which is near the reference white color for the receivers (9300.degree. K.+27MPCD).
The white balance of the white color for the video monitors in broadcasting stations should be precisely adjusted to the decided reference white color. In the broadcasting stations, various white balance adjusting methods have been employed as follows.
(1) A method of color memorization by an operator
(2) A method of using a white color balance control device
(3) A method of comparing color with the reference white light source
The first method of color memorization is not suitable method because of inferior accuracy and individual difference by operators.
The second method of using the white color balance control device is superior to the first method however, it has the disadvantages that a reference tube having the precise reference white color is used, and the memory module of the white color balance control device should be used depending upon the reference tubes and a routine calibration is needed and the maintenance is not easy whereby it is difficult to adjust white balance at desired times by the white color balance control device.
The third method of comparing with the reference white light source is effective for color adjustment in high accuracy in one sight color comparison however, when the monitor and the reference white light source are not in one sight but they are departed, this method has disadvantages that the accuracy for color adjustment is inferior and the reference white light source is usually expensive and the routine calibration is needed and the maintenance is not easy and the white color balance adjustments by this method at desired times are not easy. Accordingly, the third method is also not suitable for the white color balance adjustment.
The reference white light source used in the third method is prepared by combination of a light source and suitable filter, whereby the luminous spectrum is usually quite different from that of the white color of the color cathode ray tube for a video monitor.
In usual, when two light sources having different each luminous spectrum are combined to give an equivalent color, the accuracy of the color is remarkably inferior to that of the combination of light sources having the same luminous spectrum. Accordingly, the accuracy is not so high in the third method of white balance adjustment by using the reference white light source.
As described above, the conventional white balance adjustments have certain disadvantages and they are not optimum. It has been desired in the broadcasting fields to attain a white balance adjustment to the predetermined reference white color in high accuracy with a simple operation and by an easy operation.